PRAGUE â€” The United States and Russia opened what they called a new era in their tumultuous relationship on Thursday as they signed an arms control treaty and presented a largely united front against Iranâ€™s nuclear program, marking a sharp change since they broke over the Georgia war two years ago.

Obama, Medvedev sign treaty to reduce nuclear weapons

In a ceremony filled with flourish and the echoes of history, President Obama and President Dmitri A. Medvedev put aside the tensions of recent years to seal the New Start pact paring back their nuclear arsenals. The two leaders used the moment to showcase their growing personal relationship and a mutual commitment to cooperation on a host of issues.

The celebratory mood in the majestic, gilded hall of Prague Castle masked stubborn divisions on matters like missile defense and European security. Mr. Obama avoided any public criticism of Russiaâ€™s human rights record. And while they resolved to seek even deeper cuts in nuclear weapons, such an agreement would be much harder to reach than the one they signed Thursday.

The overthrow of the government in Kyrgyzstan likewise could quickly test the new bonds proclaimed in Prague given that the two countries have vied for influence there in recent years. As both sides struggled to figure out what the violent uprising would mean, the United States took a cautious approach while Russia embraced the new government and a senior official in Mr. Medvedevâ€™s delegation told reporters that Moscow still wanted an American base in Kyrgyzstan closed.

But harmony was the message of the day. â€œWhen the United States and Russia are not able to work together on big issues, itâ€™s not good for either of our nations, nor is it good for the world,â€ Mr. Obama said. â€œTogether, weâ€™ve stopped that drift, and proven the benefits of cooperation. Today is an important milestone for nuclear security and nonproliferation, and for U.S.-Russia relations.â€

Mr. Medvedev called the treaty â€œa truly historic eventâ€ that would â€œopen a new pageâ€ in Russian-American relations. â€œWhat matters most is that this is a win-win situation,â€ he said. â€œNo one stands to lose from this agreement. I believe that this is a typical feature of our cooperation. Both parties have won.â€

The Russian signaled support for the American-led drive to impose new sanctions on Iran, saying that Tehranâ€™s nuclear program had flouted international rules. â€œWe cannot turn a blind eye to this,â€ Mr. Medvedev said, while adding that sanctions â€œshould be smartâ€ and avoid hardship for the Iranian people.

Mr. Medvedev said he â€œoutlined our limits for such sanctionsâ€ to Mr. Obama in their private talks, without elaborating. Sergei Ryabkov, the deputy Russian foreign minister, said later that Mr. Medvedev supported sanctions â€œthat are targeted, that are tailored,â€ and opposed an embargo on refined oil products because it would be â€œa huge shock for the whole society.â€

The friendly tone stood in contrast to the rupture between Washington and Moscow after Russiaâ€™s war with its tiny neighbor of Georgia in 2008, when President George W. Bush shelved a civilian nuclear cooperation agreement in protest and supplied financial aid to the Georgians. Neither president mentioned Georgia in public on Thursday or the broader issue of Russiaâ€™s assertiveness with its neighbors.

The two played down their quarrel over American plans to build missile defense in Europe, despite recent comments by Russian officials threatening to withdraw from the treaty if the United States pressed too far. And Mr. Obama expressed no public concern about Russian authoritarianism, a topic that routinely flavored discussions during Mr. Bushâ€™s presidency, and even he was sometimes criticized for not raising it more strenuously.

Mr. Obama and Mr. Medvedev smiled and whispered with each other as they sat side by side signing the treaty. Mr. Obama called his counterpart a â€œfriend and partnerâ€ and said â€œwithout his personal efforts and strong leadership, we would not be here today.â€ For his part, Mr. Medvedev said the two had developed a â€œvery good personal relationship and a very good personal chemistry, as they say.â€

White House officials described the relationship in effusive terms. â€œWeâ€™re having a real conversation,â€ said Michael McFaul, the presidentâ€™s Russia adviser. â€œWeâ€™re not reading talking points.â€ Robert Gibbs, the White House press secretary, said Mr. Obama â€œgenuinely feels like they can sit down and call each other and work through a series of issues in a very frank and honest way.â€

Russian officials likewise expressed optimism that was absent from such meetings not long ago. â€œOur mutual trust was below zero,â€ said Mikhail Margelov, chairman of the foreign affairs committee in the upper house of Parliament. â€œNow we have to correct the mistakes of the past and move forward.â€

Under the treaty, if ratified, each side within seven years would be barred from deploying more than 1,550 strategic warheads or 700 launchers. Because of counting rules and past reductions, neither side would have to eliminate large numbers of weapons to meet the new limits. But the treaty re-establishes an inspection regime that lapsed in December and could serve as a foundation for deeper reductions later.

The rapprochement worries many in a region once dominated by Moscow. The cover of the influential Czech weekly Reflex showed Mr. Obama kissing Leonid Brezhnev, along with the warning, â€œdangerous kisses with Moscow.â€ The leading Polish newspaper Gazeta Wyborcza ran a snide commentary titled â€œObama is coming, but itâ€™s no longer our Obama.â€

Lubos Dobrovsky, a former Czech defense minister who presided over the dissolution of the Warsaw Pact, said he feared that Mr. Obama was appeasing Russia. â€œThis treaty is a diplomatic and military victory for Moscow,â€ he said in an interview, â€œand I am not happy that this American defeat is being showcased in Prague.â€

Hoping to soothe such concerns, Mr. Obama spoke by phone with President Mikheil Saakashvili of Georgia before leaving Washington and then hosted 11 leaders from the region here for a dinner of devilâ€™s fish, scallops and California wine.

â€œHe gave us reassurances that we are not in a vacuum, that we are anchored in Europe and NATO, that we belong somewhere,â€ Prime Minister Jan Fischer of the Czech Republic said in an interview afterward. But history is hard to ignore, he added. â€œThe people of the Czech Republic will be viewing relations with Russia through the rear view mirror, but we need to look through the front screen, which is much larger.â€

Prime Minister Donald Tusk of Poland told reporters that he asked Mr. Obama directly how the renewed Russian-American ties â€œmay affect the security of countries in the region,â€ and added that â€œwe received assurances on the part of the United Statesâ€ that its commitment to its partners here remained undiminished.